The invention relates to an ohmic contact on a compound semiconductor layer, and more particularly to a non-alloyed ohmic contact having a super-lattice structure for a compound semiconductor device.
It is well known that parasitic resistances deteriorate the overall performances of semiconductor devices. One of the important causes of the parasitic resistances is the contact resistance such as a Schottky barrier which appears at the metal-semiconductor interface. The reduction of the potential barrier height is important to allow semiconductor devices exhibit such excellent performances as high speed and high frequency performances, particularly for submicron devices which utilize excellent properties of heterostructures such as HEMT (high electron mobility transistor) and HBT (heterojunction bipolar transistor). The scaling down of the device are being on the increase for an improvement in the performance and a high density of the integrated circuits. The importance and requirement of the reduction of the potential barrier or the contact resistance at the metal-semiconductor interface are on the increase, as both the scaling down of the device and thus the reduction of the device dimension and the improvements in the high speed and high frequency performances are increasingly required. Ohmic contacts are well known as an ideal contact free of the potential barrier and thus the metal-semiconductor contact resistance. Alloyed ohmic contacts have problems in the vertical and lateral diffusion of contact metal due to a high temperature processing. As the scaling down of the device is improved, such problems with the alloyed ohmic contacts are more considerable. That is why non-alloyed ohmic contacts tends to take an extremely important role. Some types of non-alloyed ohmic contacts on the compound semiconductor layer have been proposed. Typical non-alloyed ohmic contacts on the compound semiconductor layer such as GaAs layer will be described as the prior art.
Low resistance non-alloyed ohmic contacts on an n-type GaAs layer are disclosed in Journal of Vacuum Science and Technology, 1981, Vol. 19. pp. 626-627, "Ohmic contacts to n-GaAs using graded band gap layers of Ga.sub.1-x In.sub.x As gown by molecular beam epitaxy" J. M. Woodall et. al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 1.
FIG. 1 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on n-type GaAs layer. A layer 2 of GaAs doped with an n-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6 of n-type Ga.sub.1-x In.sub.x As is grown by molecular beam epitaxy on the n-type GaAs layer 2. The n-type Ga.sub.1-x In.sub.x As layer 6 is graded in composition from .times.=0 at the GaAs interface to 0.8.ltoreq..times..ltoreq.1 at opposite interface. A layer 4 of InAs doped with n-type dopant is formed on the n-type Ga.sub.1-x In.sub.x As graded layer 6. A metal contact 5 is formed on the n-type InAs layer 4. From FIG. 1, it is understood that the conduction band profile of the contact across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the n-type Ga.sub.1-x In.sub.x As graded layer 6. This results in a nearly zero Schottky barrier height at the interfaces between the metal contact 5 and the n-type Ga.sub.1-x In.sub.x As graded layer 4 Thus, such non-alloyed low resistance ohmic contact to the n-type GaAs layer is obtained.
Further, another type of low resistance non-alloyed ohmic contacts on an n-type GaAs layer is disclosed in Applied Physics Letters, Vol. 53, pp-900, 1988, "Extremely low resistance non-allowed ohmic contacts on n-type GaAs using InAs/InGaAs and InAs/GaAs strained-layer superlattices", C. K. Peng, at. al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 2.
FIG. 2 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on n-type GaAs layer. A short-period n-type GaAs/InAs strained-layer superlattice 7 is grown by molecular beam epitaxy on a n-type GaAs layer 2. A layer 4 of n-type InAs is formed on the n-type GaAs/InAs strained-layer superlattice 7. A metal contact 5 is formed on the n-type InaAs layer 4.
The strained-layer superlattice 7 comprises alternating n-type GaAs and n-type InAs layers. Each InAs layer forms a quantum well and each GaAs layer forms potential barrier. Electrons serving as carriers are confined in the periodical potential wells in the conduction band. In the superlattice, the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of electrons through the potential barrier occurs. Electron currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The electrons exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained.
Another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer is disclosed in Electronics Letters 16th Aug. 1990 Vol. 26, No. 17, "Very low resistivity ohmic contact to p-type GaAs using In.sub.x Ga.sub.1-x As interlayer", P. L. Janega at al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 3.
FIG. 3 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A layer 2' of GaAs doped with a p-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6' of p-type Ga.sub.1-x In.sub.x As is grown by molecular beam epitaxy on the p-type GaAs layer 2'. The p-type Ga.sub.1-x In.sub.x As layer 6' is graded in composition from .times.=0.07 at the GaAs interface to 0.5 at opposite interface. A layer 4' of InAs doped with p-type dopant is formed on the p-type Ga.sub.1-x In.sub.x As graded layer 6'. The doping level of the p-type InAs layer is 1.times.10.sup.19 atom/cm.sup.-3. A metal contact 5 is formed on the n-type InAs layer 4. From FIG. 3, it is understood that the valence band profile across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Holes serving as carriers across the interfaces of the layers experience a nearly zero potential barrier. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the p-type Ga.sub.1-x In.sub.x As graded layer 6'. This results in a 0.4 eV to 0.5 eV of the Schottky barrier height at the interfaces between the metal contact 5 and the p-type InAs layer 4'. Thus, such non-alloyed low resistance ohmic contact to the p-type GaAs layer is obtained.
Further, another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer has been known. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 4.
FIG. 4 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A short-period p-type GaAs/InAs strained-layer superlattice 7' is grown by molecular beam epitaxy on a p-type GaAs layer 2'. A layer 4' of p-type InAs is formed on the p-type GaAs/InAs strained-layer superlattice 7'. A metal contact 5 is formed on the p-type InAs layer 4'.
The strained-layer superlattice 7' comprises alternating p-type GaAs and p-type InAs layers. Each InAs layer forms a quantum well and each GaAs layer forms potential barrier. Holes serving as carriers are confined in the periodical potential wells in the valence band. In the superlattice structure 7', the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of holes through the potential barrier occurs. Hole currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The holes exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained.
Another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer is disclosed in IEEE Electron Devices Letters 1987 Vol. 8, No. 30, "An(Al,Ga)As/GaAs Heterostructure Bipolar Transistor with Nonalloyed Graded-Gap Ohmic Contacts to the Base and Emitter", M. A. Rao at al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 5.
FIG. 5 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A layer 2' of GaAs doped with a p-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6" of p-type GaAs.sub.1-x Sb.sub.x is grown by molecular beam epitaxy on the p-type GaAs layer 2'. The p-type GaAs.sub.1-x Sb.sub.x layer 6" is graded in composition from the GaAs interface to opposite interface. A layer 4" of GaSb doped with p-type dopant is formed on the p-type GaAs.sub.1-x Sb.sub.x graded layer 6". The doping level of the GaSb layer 4" is 1.times.10.sup.19 cm.sup.-3. A metal contact 5 is formed on the n-type GaSb layer 4". From FIG. 5, it is understood that the valence band profile across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Holes serving as carriers across the interfaces of the layers experience a nearly zero potential barrier. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the p-type GaAs.sub.1-x Sb.sub.x graded layer 6". This results in a 0.1 eV of the Schottky barrier height at the interfaces between the metal contact 5 and the p-type GaSb layer 4". Thus, such non-alloyed low resistance ohmic contact to the p-type GaAs layer is obtained.
Further, another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer has been known. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 6.
FIG. 6 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A short-period p-type GaAs/GaSb strained-layer superlattice 7" is grown by molecular beam epitaxy on a p-type GaAs layer 2'. A layer 4" of p-type GaSb is formed on the p-type GaAs/GaSb strained-layer superlattice 7". A metal contact 5 is formed on the p-type GaSb layer 4".
The strained-layer superlattice 7" comprises alternating p-type GaAs and p-type GaSb layers. Each GaSb layer forms a quantum well and each GaAs layer forms potential barrier. Holes serving as carriers are confined in the periodical potential wells in the valence band. In the superlattice structure 7", the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of holes through the potential barrier occurs. Hole currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The hole exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained.
The above ohmic contact using either the InGaAs or GaAsSb graded layer of p-type or n-type are, however, engaged with the following disadvantages which will be investigated.
As described the above, the composition-graded layer providing the low contact resistance and thus a nearly zero Schottky barrier is grown by molecular beam epitaxy. The growing process of the composition-graded layer requires an extremely precise and continuous control of the flux intensity of a molecular beam for growing the composition-graded layer. That is why it is difficult to grow the available composition-graded InGaAs or GaAsSb layer by the molecular beam epitaxy. Particularly, it is extremely difficult to control the flux intensity of a molecular beam of Group-V elements such as As and Sb. The inferiority of the control of the flux intensity of a molecular beam results in an abrupt discontinuity in compositions of the composition-graded layer. Such abrupt discontinuity in the composition causes both high density stacking faults and high density misfit dislocations. This results in an enlargement of the contact resistance. If the over-shoot of the In flux appears in the latter half of the growing process of the composition-graded layer, the flux ratio of Group-V elements to Group-III elements are varied from the design value. This results in a roughness of the surface of the composition-graded layer, in addition the inferiority of the crystal quantity. This causes the enlargement of the contact resistance, for which reason it is no longer possible to obtain any excellent ohmic contact. Although the composition-graded layer is theoretically able to provide an ohmic contact between the metal and compound semiconductor layer, it is actually difficult to provide a desirable low resistive ohmic contact in the high yield and the high reliability due to the difficulty in the control of the flux intensity of composition for the growing of the composition-graded layer by the molecular beam epitaxy.
If the organic metal chemical vapor deposition is used for growing the composition-graded layer in replacement of the molecular beam epitaxy, such problems as the above in the appearances of the staking faults and the misfit dislocations which causes the enlargement in the contact resistance. Therefore, similarly to the molecular beam epitaxy, the organic metal chemical vapor deposition is also not available to grow a high quality composition-graded layer for providing a desirable low resistance ohmic contact.
Consequently, the composition-graded layer has the above mentioned problem in the inferiorities of the low contact resistance, the reliability and the yield of the ohmic contact.
The above ohmic contact using either the InAs/GaAs or GaAs/GaSb strained-layer superlattice of p-type or n-type are also engaged with the following disadvantages which will be investigated with reference to FIG. 7.
As illustrated in FIG. 7, the GaAs/InAs strained-layer superlattice 7 is formed on the GaAs buffer layer 2 overlying a semiconductor substrate 1. The InAs layer 4 overlays the GaAs/InAs strained-layer superlattice 7. The metal contact 5 is formed on the InAs layer 4. As described the above, the GaAs/InAs strained-layer superlattice 7 is grown by the molecular beam epitaxy. In this case, the growing process of the superlattice 7 by the molecular beam epitaxy is free from such difficulty as those in the precise and continuous control of the flux intensity of molecular beams. Namely, the growing process for the superlattice 7 has a facility as compared to the growing process for the composition-graded layer.
According to the Heisenberg's uncertainty principle, it is impossible to specify a precise position of electrons or holes. The electron's position may thus be represented as a wave function of carriers and thus electrons and holes. The superlattice 7 has such a short period of the potential well and the potential barrier as to allow the resonant tunneling between adjacent potential wells. Actually, such short period of the superlattice 7 is generally smaller than a wave length of electrons. Such short period of the superlattice 7 also has respective width constancies of the potential barriers and potential wells. Although the carriers and thus electrons or holes experience a nearly zero potential barrier but only within the superlattice structure, the carriers and thus electrons or holes experience a relatively large potential barrier both at the interface between the superlattice structure 7 and the GaAs buffer layer 2 and at the interface between the superlattice structure and the InAs layer. Physically, the carriers and thus electrons or holes experience such a conduction band profile across the interfaces as that illustrated in FIG. 8 rather than the conduction band profile illustrated in FIG. 2. In this case, the effective energy band profile experienced by electrons is near an energy band profile as illustrated in FIG. 8 across the n-type GaAs layer 2 underlying an n-type In.sub.0.5 Ga.sub.0.5 As layer 10 underlying the n-type InAs layer 4 underlying the metal contact 5. The constancy of such short periodical superlattice provides the discontinuity in the conduction band at the superlattice structure. This results in an enlargement of the contact resistance.
Further, the ohmic contact using the superlattice structure has another problems both in the misfit dislocation and in the staking fault due to the lattice mismatch. A lattice constant of InAs is larger by 7% than a lattice constant of GaAs. Such discontinuity of the lattice constants of the InAs/GaAs layers in the superlattice causes the lattice mismatch providing the misfit dislocation and the staking fault in the superlattice structure. The misfit dislocation and the staking fault form space charge regions around thereof. The space charge region due to a high density of the misfit dislocation and the staking fault in the superlattice structure results in a large internal resistance.
Similarly, such misfit dislocation and the staking fault due to the lattice mismatch also appears both at the interface between the GaAs/InAs superlattice structure and the GaAs layer and at the interface between the GaAs/InAs superlattice structure and the InAs layer. Such misfit dislocation and the staking fault form space charge regions around thereof. The space charge region due to a high density of the misfit dislocation and the staking fault in the superlattice structure results in a large internal resistance.
Of course, the p-type GaAs/InAs superlattice also has the above mentioned problems. Further, a difference in the lattice constants between GaAs and GaSb is 8%. Such discontinuity of the lattice constants of the InAs/GaAs layers in the superlattice causes the above problems.
It is therefore required to provide a novel ohmic contact structure on a compound semiconductor layer for a semiconductor device.